Lower Jurassic

In the Lower Jurassic, the marine benthos consisted mostly of invertebrates; bivalves, gastropods and crustaceans are the commonest groups. These three groups all show major evolutionary advances when compared with their ancestors of the Palaeozoic. More bivalve taxa had developed siphons which enabled them

Fig. n. Geography of the North Atlantic region during the Middle Jurassic

to burrow more efficiently; this was either for protection or for more efficient deposit feeding. Other bivalves, like the pectinids, had become adept at clapping their valves so they could either swim through the water by jet-propulsion or rest on the sea floor. The bivalve oysters, which appeared in the Triassic, were either cemented by one valve on a hard substrate, or (like the coiled Gryphaea) lay free upon muddy or sandy sea floors.Most gastropods retained their roles as grazers, detritus feeders and scavengers. The crustaceans did not always have thick enough skeletons to be well preserved, but the minute bivalved ostracodes are sometimes abundant. Nonetheless, many types of crustacean burrows are common; Jurassic marine sediments contain burrows comparable in many respects to those produced by modern crustaceans.

Many Jurassic marine communities are connected closely with a certain type of sediment; this is known as substrate dependence. It is, notable that from the Mesozoic onwards many more communities were related to the substrate than appears to have been the case in the Palaeozoic, and many of the Jurassic communities described here are therefore named after the sediment in which they occurred.

Not only the type of sediment but also the rate of deposition controls the benthos in modern environments; many animals are unable to adapt to rapid sedimentation rates, especially when these conditions are linked with the presence of unstable substrates. The communities described in the following sections also show changes probably caused by a lower food supply, which, in modern environments, generally decreases with distance from the shore. Communities are also affected by the degree of oxygenation of the sea water near the sea floor; poorly aerated bottom conditions severely limit the variety of benthos and help to preserve organic matter in the sediment leading perhaps to the formation of bituminous and laminated shales.

Genera of nekton (the swimming animals) and plankton (the floating animals and plants) are in general more widely distributed in the Jurassic than are benthic genera. They were not affected by the nature of the sea floor, and wide oceans did not form a barrier to their migration; for example, jsome individual ammonite genera in the early Jurassic have almost a world-wide distribution. This is a reflection of the greater homogeneity of pelagic environments in contrast to the more heterogeneous mosaic-like environments found on the sea floors of shelf areas. The ammonites, in particular, have now been studied for well over a century, and because of their wide geographical distribution and generally robust shells they are pre-eminently suitable for stratigraphical correlation over large areas and between different marine environments. The Jurassic ended around 135 million years ago (Lambert, 1971) and the rocks deposited during this period have been divided into 64 zones based on the detailed study of ammonite evolution and the appearance of new genera and species. Thus, on average, each zone represents about 1.2 million years.

In the Lower Jurassic, the ammonite zones appear to be of two types; they are either characterized by successive species of the same genus or family (when the time divisions are thus directly related to rates of evolution), or by the sudden appearance of genera whose detailed ancestry is poorly known (in these latter cases, correlation is based on the assumption that migration has been rapid). Details of the ammonite species in each Lower Jurassic zone have been recorded by Dean et al. 1961.

Before the ammonites had been studied in detail, the early nineteenth century geologists divided up the beds entirely by the type of rock present — this was the approach adopted by William Smith, the canal engineer (1769—1839), who first recognized that strata could be identified by their fossils. The Lower Jurassic of Britain was named the Lias after the west country term for flat stones (Arkell, 1933), the Lias being distinguished from the beds above on the grounds of lithology; the clays and sands of the Lias are clearly distinct from the succeeding shelly oolitic limestones of the Middle Jurassic of England.

The Lias of England consists mostly of clay and muddy limestone, but sands and ironstones are also present at some levels, with oolitic ironstone formations being particularly characteristic of the Middle Lias. The Lower, Middle and Upper Lias are not equal divisions; in northern Europe they have been divided respectively, into twelve, two and six ammonite zones.

The ammonites have one major failing as fossils for correlation: they were very sensitive to changes in salinity, and as a result they are absent or very rare in those areas that were partially isolated from the open sea. Many other planktonic and nektonic invertebrates were also stenohaline; benthic invertebrates included many animals which were stenohaline, but it seems from analogy with their living descendants that some benthos were euryhaline, notably the oysters, which often provided the only faunal indication of marine influence in saline or fresh water lagoon environments. In Britain, the broadly lagoonal environments of the late Triassic were followed by general marine conditions which extended across much of northern Europe (Fig. n), and ammonites are common in most Jurassic beds.

The Jurassic nekton included fish and reptiles but they are seldom a dominant element of the total fauna. Thus, in most of the succeeding reconstructions, the abundant invertebrate organisms have been given pride of place.

63 Bituminous Mud Community

This is a reconstruction from a bituminous shale. Probably, few of the animals lived on the sea floor and the assemblage is dominated by nekton and plankton: cephalopods (ammonites, nautiloids and belemnites), and marine vertebrates (ichthyosaurs, pliosaurs and fish) which were scavengers and carnivores. The fauna also in-

eludes filter feeders (bivalves, crinoids, serpulids and barnacles) that were attached to floating vegetation, wood and debris projecting above the anaerobic sea floor.

The sediment was rich in organic matter derived from the partially decomposed plant and animal remains trapped in the fine mud accumulating on the oxygen-deficient sea floor. Bituminous shales formed in two distinct environments during the Lias. The first was related to very shallow, but calm, water in iagoonal' conditions where the sea floor was anaerobic owing to its partial isolation from the open sea. The second type formed in the deepest parts of marine basins where normal nekton and plankton were present in the main body of sea water, but where limited water circulation allowed stagnant conditions to develop on the sea floor. The fossils in the first type of shale consist only of limited numbers of euryhaline benthos, like those seen in the basal (pre-ammonite) beds of the Lower Lias. The example shown is the second type. The cephalopods present were stenohaline, and probably lived in normal sea water well above the stagnant bottom. The sea was probably around 200 to 300m deep; comparable but deeper water, oxygen-poor environments occur today in the Black Sea, the Gulf of California (Calvert, 1964), and the Mediterranean.

Although benthic animals are usually absent in the deeper type of bituminous shale, some bedding planes may be crowded with bivalves, crinoids, or other animals that are normally benthic. A clue to this anomaly is provided by large logs of fossil wood, which are often found encrusted with large numbers of attached organisms (Seilacher et al, 1968; Hauff, 1953). On any one log, the attached animals were usually all of one species, but in our reconstruction we have included several kinds in order to show some of the varied genera which can occur on logs in the Lias. The mode of life of Bositra [Posidonia], is based upon the interpretation of Jefferies and Minton (1965) but there is considerable doubt about the plausibility of swimming bivalves of this type, although this theory would help to explain the wide distribution of Bositra in sediments that often lack any other benthos.

The preservation of calcite and aragonite shells is often unusually good in bituminous shales. The impermeable nature of the sediment has often prevented solution or recrystallization of the calcium carbonate, but precipitation of pyrite (iron sulphide) has often occurred in open cavities and chambers within the shells. Sometimes the pyrite has prevented the shells from being flattened when the sediment was compacted by the weight of younger strata. In the stagnant water of this environment, neither action of currents nor of burrowers could dismember the reptile skeletons. Some ichthyosaurs have been found with their stomach contents preserved in place; these may include fish remains and the chitinous

Fig. 63 Bituminous Mud Community a Dactylioceras (Mollusca: Ammonoidea: Cephalopoda) b Bositra (Mollusca: Bivalvia: Pterioida) c Pentacrinus (Echinodermata: Crinozoa)

d belemnite (Mollusca: Cephalopoda: Coleoidea) e Oxytoma (Mollusca: Bivalvia: Pterioida)

f Inoceramus (Mollusca: Bivalvia: Pterioida) g Gervillia (Mollusca: Bivalvia: Pterioida)

h serpulids (Annelida — polychaete)

i ichthyosaur (Vertebrata: Reptilia: Euryapsida)

j Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

Fig. 63 Bituminous Mud Community a Dactylioceras (Mollusca: Ammonoidea: Cephalopoda) b Bositra (Mollusca: Bivalvia: Pterioida) c Pentacrinus (Echinodermata: Crinozoa)

d belemnite (Mollusca: Cephalopoda: Coleoidea) e Oxytoma (Mollusca: Bivalvia: Pterioida)

f Inoceramus (Mollusca: Bivalvia: Pterioida) g Gervillia (Mollusca: Bivalvia: Pterioida)

h serpulids (Annelida — polychaete)

i ichthyosaur (Vertebrata: Reptilia: Euryapsida)

j Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

hooks from belemnite arms. These same reptiles occasionally contain juveniles, presumably their young, which suggests that they were viviparous. The reptiles' skins have often been preserved, the hides having been tanned by chemicals in the anaerobic substrate.

The bituminous shale community illustrated is based upon the fauna from the Upper Lias of Yorkshire; both deep and shallow types of assemblage are found in the Lias of Yorkshire, central England and the Glamorgan coast and in the Blue Lias, 'Shales with Beef, Black Ven and Belemnite Marls of the coast of Dorset, England.

64 Restricted Clay Community

Since the sea floor was not stagnant, this community contains many burrowing animals in addition to the nektonic and planktonic cephalopods, reptiles and fish shown previously (Fig. 63). The fossil benthos is dominated by detritus feeders: protobranch bivalves (Nucula and Nuculana — the latter is distinguished by its pointed posterior), lucinoid bivalves (Lucina, Mactromya), and procerithid gastropods. These animals were all able to cope with the soft substrate which prevented many other burrowers from colonizing the area. Crustacea and worms also burrowed in this environment; they are recognizable from their burrows. One branching worm-burrow (Chondrites) is visible when it has been filled with slightly different mud from the surrounding sediment, and some simpler tubular burrows may be preserved in pyrite.

The burrowing activity obliterated the bedding laminae seen in the more bituminous muds, so many metres of sediment accumulated with no marked bedding being preserved. The sediments lack ripples and scour structures, and probably accumulated in depths greater than 30m, below the limits of wave motion and strong current activity.

The burrowing action of the protobranchs and the worms appears to have made the sea floor too soft and unstable for the successful development of a more diverse benthos. A careful search of the sediment can reveal an abundance of minute bivalves which were the shells of juveniles of other genera which are typical of less muddy shallower water environments. In the restricted clay environment the spat of these genera settled on unfavourable muds and failed to achieve adulthood.

The reasons why this habitat was successfully colonized by protobranchs and lucinoids in the Jurassic can be discerned from studies of their present day descendants. Modern protobranchs are shallow burrowers, living just below the sediment/water interface as mobile or semi-sessile detritus feeders. The lucinoids construct b

Fig. 64 Restricted Clay Community

g

Gervillia (Mollusca: Bivalvia: Pterioida)

a

Dactylioceras (Mollusca: Cephalopoda: Ammonoidea)

h

Chondrites (Trace fossil: annelid burrow)

b

belemnite (Mollusca: Cephalopoda: Coleoidea)

i

Inoceramus (Mollusca: Bivalvia: Pterioida)

c

crinoid (Echinodermata: Crinozoa)

j

pectinid (Mollusca: Bivalvia: Pterioida)

d

Procerithium (Mollusca: Gastropoda: Mesogastropoda)

k

Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

e

Oxytoma (Mollusca: Bivalvia: Pterioida)

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